The reforming of hydrocarbons is a well-known process which is practised on an industrial scale. It has been suggested that diesel fuel may be reformed to form hydrogen, and that the hydrogen may be added to diesel fuel or biodiesel to directly reduce NOx and smoke (particulates) emissions from a diesel engine. As an alternative, or in addition, the hydrogen present in reformate may assist the catalytic aftertreatment of the lean (that is, oxygen-rich) exhaust gases from diesel engines, especially in the reduction of NOx to N2. See Energy & Fuels 2003, 17, 1464-1473 and Journal of Catalysis 208, 435-447 (2002). Other theoretical reviews of the possibilities of this technology include SAE 2005-01-2087 A. Tsolakis et al and Chemical Engineering Journal 117 (2006) 131-136. Our GB 2462798 also contemplates the use of an exhaust gas reformer in a light duty diesel engine.